1. Field of the Invention
This invention relates to a continuous process for the production of a polyisocyanate.
2. Description of the Prior Art
The method of producing a polyisocyanate by reacting an organic diisocyanate with an adduct-producing agent which reacts with an isocyanate group to form a urea bond and a gas is known. Examples of the organic diisocyanate are alicyclic diisocyanates and aliphatic diisocyanates, such as hexamethylene diisocyanate. Examples of the adduct-producing agent are water, monovalent tertiary alcohol, formic acid, hydrogen sulfide and organic primary monoamine. Generally, the organic diisocyanate is used in excess with respect to the adduct-producing agent, for example, at least about 3 mols, preferably at least about 10 mols of the organic diisocyanate is used per mol of the adduct-producing agent.
The reaction of the organic diisocyanate with the adduct-producing agent is characterized by the formation of a urea bond and generation of a gas, as is shown in the following reaction schemes: ##STR1##
The urea bond thus-formed further reacts with an isocyanato group to form a 3 or more functional organic polyisocyanate. For example, a urea dimer reacts with a diisocyanate monomer to form a trifunctional polyisocyanate trimer. The resulting polyisocyanate is typically either a viscous liquid or a solid. However, such a solid polyisocyanate is highly soluble in common organic solvents such as toluene, xylene and acetic acid esters.
As shown above, the reaction between the organic diisocyanate and adduct-producing agent is consecutive, but the urea dimer or urea bond-containing oligomers formed in the course of the reaction are solids, and are so low in miscibility with the reaction mixture and the polyisocyanate product that they can form a precipitate that blocks the production equipment or produces a turbid liquid polyisocyanate. Thus, it is undesirable that a urea bond remains not further reacted with an isocyanate group. Even a polyisocyanate free from urea bonds is highly viscous if its molecular weight is great, and such viscous polymer is so low in miscibility with other resins or solvent that it cannot be used in combination with a polyhydroxy compound as a paint without adversely effecting the film-forming properties and physical properties of the paint. To produce low molecular weight polyisocyanate free from urea bonds, the conventional method uses the organic diisocyanate in excess with respect to the adduct-producing agent, as described above, but the use of a great excess of the organic diisocyanate, (e.g., more than 40 mols per mol of the adduct-producing agent) has its own problems, viz., not only must the excess diisocyanate be recovered, but also the organic diisocyanate used enters into thermal polymerization to produce a strongly colored polyisocyanates including, for instance, uretidion rings, isocyanurate rings, and/or carbodimide bonds. To prevent these disadvantages, the organic diisocyanate is used generally in an amount of from 5 to 40 mols, and preferably from 10 to 30 mols, per mol of the adduct-producing agent.
The reaction between the organic diisocyanate and adduct-producing agent is conventionally performed in a batchwise reactor. The batchwise reaction is characterized by a uniform residence time of the reactants in the reactor which is desirable for providing a polymer having a narrow range of molecular weight. However, a batchwise reaction performed on an industrial scale is inefficient, because it involves a complex and troublesome operation that may lead to improper sequence of operations and/or an accident. Furthermore the quality of the polyisocyanate produced by the batchwise reaction differs from batch to batch.
However, attempts to perform the reaction between the organic diisocyanate and adduct-producing agent in a continuous manner involve the following problems: (a) Even if all stages of the reaction are performed in a single continuous stirring tank reactor to provide a homogeneous mixture, it is practically impossible to keep the residence time of the mixture of reactants uniform. In other words, part of the liquid mixture of reactants may reside in the reactor for only a short period of time and another part resides for a longer period, thus producing a by-product that has an unreacted urea group or a product having a very large molecular weight. It is thus difficult to keep producing a polyisocyanate of high quality by the continuous process. (b) If all stages of the reaction are performed in a pipe reactor that provides a perfect plug flow or through a series of connected continuous stirring tank reactors to provide a substantially plug flow, the residence time in theory can be maintained uniformly, and the above problems seem avoidable. However, in fact, even a pipe reactor cannot provide a perfect plug flow in a reaction of the type contemplated by this invention that involves the generation of a gas, because the gas generated causes turbulence or convection in the liquid reaction mixture. A plurality of partitions such as baffles may be used to divide the pipe reactor into compartments, but local residence of the reaction mixture or uneven distribution of temperature in the initial period of the reaction unavoidably results in the formation of an unreacted urea compound or a polymer. Connecting continuous stirring tank reactors is effective for preventing the turbulence of the liquid reaction mixture due to gas generation, but the use of a plurality of continuous stirring tank reactors for providing a substantial plug flow is not an economical method to adopt on an industrial scale.